1. Field of the Invention
The present invention relates to a compressor, and particularly to a method and an apparatus for detecting the quantity of a mixture of coolant and lubricant contained in the compressor.
2. Description of the Prior Art
Heat pumps used as heating sources and the heat absorbing sources of freezers employ gaseous coolant such as Freon that is easy to liquefy. In the heat pump, a compressor compresses the coolant into a high-temperature high-pressure gas, and a condenser removes heat from the coolant to liquefy the same. Thereafter, an expander expands the liquefied coolant, and an evaporator lets the coolant absorb heat. The coolant then becomes a low-pressure gas to complete a freezing cycle.
During the stoppage of operation of the compressor of the heat pump particularly in a low ambient temperature, for example in winter, the gaseous coolant may liquefy and flow back into the compressor. IF this happens, the liquefied coolant mixes with lubricant contained inside the compressor, thereby diluting the lubricant. If the dilution is excessive to soften the viscosity of the lubricant, the lubricant may not sufficiently function and cause locking, abrasion, etc., on rotating and wobbling parts of the compressor mechanism.
To avoid such trouble, the lubricant in the compressor is usually preheated before starting the heat pump to operate. This will be explained with reference to FIG. 1. The figure shows a vertical-closed-type rotary compressor. The compressor comprises a closed casing 1 which accommodates a motor 4 and a compressor mechanism 5 connected to a lower part of the motor 4. The motor 4 comprises a rotor 2, a stator 3, and a stator coil 3c. A low pressure coolant gas is sucked through a suction pipe 6, and a high-pressure coolant gas is discharged through a discharge pipe 7. The motor 4 receives electric power through terminals 8 and lead wires 9. The closed casing 1 contains lubricant 10 that is at a low level L in the figure. A preheater coil 11 is wound around a lower part of the casing 1 where the lubricant 10 is stored.
FIG. 2 shows an alternative of the heater. In this figure, a heater 12 and a heater case 13 enclosing the heater 12 are inserted into a lower part of the closed casing 1.
FIG. 3 shows a sequence of operations of the heat pump. Before starting the heat pump, the heater coil 11 or heater 12 preheats the lubricant 10 for a predetermined time period, thereby evaporating and removing the coolant mixed with the lubricant 10. Here, the lubricant 10 is preheated on an assumption that is contains mixed coolant at the start of the heat pump. Namely, irrespective of a light or heavy quantity of the coolant mixed with the lubricant, the preheating of the lubricant is always carried out for a predetermined time period.
This technique, therefore, wastes preheating power and time, if no coolant is mixed with the lubricant, or if only a small quantity of coolant that does not bother the function of the lubricant is mixed with the lubricant. Another problem of this technique is that the compressor of the heat pump is activated only after the termination of the preheating, thereby delaying the start of operation of the heat pump.